I am unable to setup my iPhone after flashing it

How exactly did you "flash" your iPhone?

If you erase the phone and restored the operating system correctly, the phone started up to the "hello" screen to walk you through the setup.

If needed, follow the steps in this Apple Support document to reset your phone back to factory defaults, and then start over again with a new setup.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you’re trying to bypass Activation Lock here, factory reset is not going to clear that.

If the reason for the factory reset was caused by a hardware problem, that problem likely persisted.